In low pressure and gravity die casting, the surface of each metal mould or die component, which is contacted by molten metal, is provided with a mould or die coating. Under current procedures, a ceramic-based coating is used at a thickness of from about 0.05 to 1.0 mm. The main function of the coating is to provide a degree of insulation which is intended to prevent premature solidification of the molten metal, and thereby enable the complete filling of the die cavity before solidification starts. However, the coating also is to protect the steel die surfaces from erosion or corrosion by impingement by or contact with molten metal.
Current die coating technology involves the use of a water-based suspension of ceramic particles containing a water-based binder, most commonly sodium silicate. Coating mixtures of this type need to be properly stored, while stirring and testing to prepare them for use often involves tedious procedures. The coating is applied to the sand or shot blasted surface of a die component using a pressurized air spray gun. For this, the component is preheated, typically from about 150 to 220° C., such that water is evaporated from the die surface, enabling the binder to polymerize and bond the ceramic particles together and to the die surface.
The die coatings produced with the current aqueous ceramic suspensions are highly porous. The level of porosity may range from about 30 to 60%, depending on the size and shape of the ceramic particles and the amount of binder used. High porosity gives the coating very good insulating properties. However, the strength of the coatings is limited by the strength of the binder used (about 6.9 MPa in the case of sodium silicate) and the level of porosity of the coating.
Important factors in a thermally insulating die coating are porosity and surface roughness. Also, wear resistance is important since a coating with an inadequate level of wear resistance is prone to damage in use, with a consequential reduction in its useful life-time. The sodium silicate bonded coatings, produced with the current aqueous ceramic suspensions, have a low level of wear resistance which results in them having a productive life-time of not more than about two to four 8-hour shifts. However, even during such a short life-time, production needs to be stopped from time to time to enable repair of the coating by a “touch-up” operation.
In U.S. Pat. No. 4,269,903 to Clingman et al, there is disclosed a ceramic seal coating formed on at least one of two relatively rotatable members, such as rotating air foils of an axial flow compressor. The process seeks to provide a seal coating as disclosed in U.S. Pat. No. 4,055,705 to Stecura et al, which has improved abradability. The coating of U.S. Pat. No. 4,055,705 comprises a bond coat of NiCrAlY alloy applied to a substrate and a thermal barrier which is applied over the bond layer and comprises zirconia stabilized with another oxide. The advance provided by U.S. Pat. No. 4,269,903 is in providing over that thermal barrier layer an abradable layer of porous stabilized zirconia. The porous layer is formed by thermal decomposition of organic filler material, which is co-deposited with stabilized zirconia onto the barrier layer. The co-deposition, such as by plasma spray or thermal spray process, preferably uses separate streams of organic and zirconia powders, with the organic powder chosen from a range of suitable thermoplastics. After co-deposition, the organic material is decomposed by heating, to leave an abradable zirconia layer having a porosity of from about 20 to about 33% and, hence, a suitable level of abradability. The abradable layer enables wear of at least one of two relatively rotatable components in rubbing contact such that loss of a fluid seal between the components is avoided.
In the process of U.S. Pat. No. 4,269,903 the organic material is used because, after its co-deposition with zirconia, the organic material is able to be removed by thermal decomposition to leave a porous, and hence abradable, layer of zirconia. An alternative purpose for co-deposition of organic powders with ceramic and/or metal powders is disclosed in U.S. Pat. No. 5,718,970 to Longo.
The process of U.S. Pat. No. 5,718,970 is concerned with providing a substrate with a thermally sprayed duplex coating of a plastics material which is co-deposited with a higher melting point ceramic material and/or metal. It is asserted that while metal and ceramic powders necessitate spraying with high temperature gas streams, such as plasma sprays or acetylene gas, plastic powders are usually sprayed with low temperature gas streams, such as hydrogen or natural gas, to prevent superheating and oxidation of the plastic powder. The solution for achieving a duplex coating is to use a powder comprising particles having a core of plastic material and, on the core, a substantially continuous particulate cladding of ceramic and/or metal. The cladding may be adhered to the core as a consequence of heating to soften the core, or by use of a suitable binder. The duplex coating produced by thermal spraying of such powders is able to exhibit characteristics of the ceramic and/or metal and of the polymer material, with the coating indicated as having particles of the plastic material dispersed in a continuous matrix of the ceramic and/or metal.